From childhood, he showed physical courage and ability, and became his parents' favourite.[12] He was enrolled in the 2nd Foot Guards and made a Knight of the Bath aged four.[13] He was intended, by the King and Queen, for the office of Lord High Admiral, and, in 1740, he sailed, as a volunteer, in the fleet under the command of Sir John Norris, but he quickly became dissatisfied with the Navy, and, instead secured the post of colonel of the First Regiment of Foot Guards on 20 February 1741.[14]

In 1745, Cumberland was given the honorary title of Captain-General of the British land forces and in Flanders became Commander-in-Chief of the allied British, Hanoverian, Austrian and Dutch troops despite his inexperience. He initially planned to take the offensive against the French, in a move he hoped would lead to the capture of Paris, but was persuaded by his advisors that this was impossible given the vast numerical superiority of the enemy.[17]

Jacobite satire of the Duke of Cumberland in the Highlands

As it became clear that the French intention was to take Tournai, Cumberland advanced to the relief of the town, which was besieged by Marshal Saxe. In the resulting Battle of Fontenoy on 11 May 1745, the Allies were defeated by the French.[18] Saxe had picked the battleground on which to confront the British, and filled the nearby woods with French marksmen. Cumberland ignored the threat of the woods when drawing up his battle plans, and instead concentrated on seizing the town of Fontenoy and attacking the main French army nearby. Despite a concerted Anglo-Hanoverian attack on the French centre, which led many to believe the Allies had won, the failure to clear the woods and of the Dutch forces to capture Fontenoy forced Cumberland's force onto the retreat. Following the battle Cumberland was frequently criticised for his tactics, particularly the failure to occupy the woods.[19] In the wake of the battle, Cumberland was forced to retreat to Brussels and was unable to prevent the fall of Ghent, Bruges and Ostend.[20]

As the leading British general of the day, he was chosen to put a decisive stop to Charles Edward Stuart, a direct descendant of James VII of Scotland and II of England (James VII/II was the last Stuart king on the male line), in the Jacobite rising of 1745. His appointment was popular, and caused morale to soar amongst the public and troops loyal to King George.[21]

Recalled from Flanders, Cumberland proceeded with preparations for quelling the Stuart (Jacobite) uprising. The Jacobite army had advanced southwards into England, hoping that English Jacobites would rise and join them. However, after receiving only limited support such as the Manchester Regiment, the followers of Charles decided to withdraw to Scotland.[22]

Cumberland joined the Midland army under Ligonier, and began pursuit of the enemy, as the Stuarts retreated northwards from Derby.[3] On reaching Penrith, the advanced portion of his army was repulsed on Clifton Moor in December 1745, and Cumberland became aware that an attempt to overtake the retreating Highlanders would be hopeless.[23]Carlisle was retaken, and he was recalled to London, where preparations were in hand to meet a suspected French invasion.[3] The defeat of his replacement as commander, Henry Hawley, roused the fears of the English people in January 1746, when, under a hail of pistol fire, "eighty dragoons fell dead upon the spot" at Falkirk Muir.[24]

Arriving in Edinburgh on 30 January 1746, he at once proceeded in search of Charles. He made a detour to Aberdeen,[25] where he spent some time training the well-equipped forces now under his command for the next stage of the conflict in which they were about to engage. He trained his troops to hold their fire until the enemy came within effective firing range, fire once, and then bayonet the man to the right thereby catching the enemy under their lifted sword arm.[26]

On 8 April 1746, he set out from Aberdeen for Inverness, and, on 16 April, he fought the decisive Battle of Culloden, in which the Stuart forces were completely destroyed.[22] Cumberland ordered his troops to show no quarter against any remaining Jacobite rebels (French Army personnel, including those who were British- or Irish-born, were treated as legitimate combatants). His troops traversed the battlefield and stabbed any of the rebel soldiers who were still alive.[27] When Cumberland learned that a wounded soldier lying at his feet belonged to the opposing cause he instructed a major to shoot him; when the major (James Wolfe) refused to do so, Cumberland commanded a private soldier to complete the required duty.[27]

The British Army then embarked upon the so-called 'pacification' of Jacobite areas of the highlands. All those the troops believed to be 'rebels' were killed, as were non-combatants; 'rebellious' settlements were burned and livestock was confiscated on a large scale.[28] Over a hundred Jacobites were hanged.[29] Women were imprisoned and droves of people were sent by ship to London for trial and as the journey took up to 8 months many of them died on the way.[27]

Following Culloden, Cumberland was nicknamed "Sweet William" by his Whig supporters and "The Butcher" by his Tory opponents[30] the latter being a taunt first recorded in the City of London[31] and used for political purposes in England. Cumberland's own brother, the Prince of Wales (who had been refused permission to take a military role on his father's behalf), seems to have encouraged the virulent attacks upon the Duke. Cumberland preserved the strictest discipline in his camp. He was inflexible in the execution of what he deemed to be his duty, without favour to any man. In only a few cases he exercised his influence in favour of clemency. The Duke's victorious efforts were acknowledged by his being voted an income of £25,000 per annum over and above his money from the civil list.[3] A thanksgiving service was held at St Paul's Cathedral, that included the first performance of Handel's oratorio Judas Maccabaeus, composed especially for Cumberland, which contains the anthem "See the Conquering Hero Comes".[32]

Cumberland's unpopularity, which had steadily increased since Culloden, interfered greatly with his success in politics, and when the death of the Prince of Wales brought the latter's son, a minor, next in succession to the throne, the Duke was not able to secure for himself the contingent regency. As a compromise, the regency was vested in the Dowager Princess of Wales, who considered him an enemy, but her powers were curtailed and she was to be advised by a committee of twelve men, headed by Cumberland.[35]

In 1757, the Seven Years' War having broken out, Cumberland was placed at the head of the Army of Observation, a force of German allies paid for by Britain which intended to defend Hanover from a French attack.[36] At the Battle of Hastenbeck, near Hamelin, on 26 July 1757, Cumberland's army was defeated by the superior forces of d'Estrées. Despite seemingly having the advantage towards the end of the battle, Cumberland's forces began to retreat. Within a short time discipline had collapsed, and Cumberland's army headed northwards in total disorder. Cumberland hoped that the Royal Navy might bring him reinforcements and supplies which would allow him to regroup and counterattack, but the British mounted an expedition to Rochefort instead, despite suggestions that it should be sent to aid Cumberland.[37]

By September 1757 Cumberland and his forces had retreated to the fortified town of Stade on the North Sea coast. The King gave him discretionary powers to negotiate a separate peace.[38] Hemmed in by the French, under Richelieu, he agreed to Convention of Klosterzeven, under which Cumberland's army was to be disbanded and much of Hanover was occupied by French forces, at the ZevenConvent (de), on 8 September 1757.[39]

On Cumberland's return to London he was treated badly by his father despite the fact that he had previously been given permission to negotiate such an agreement. When they met George II remarked "Here is my son who has ruined me and disgraced himself".[40] In response, Cumberland resigned all the military and public offices he held and retired into private life.[41]

Cumberland's final years were lived out during the first years of the reign of his nephew, George III, who acceded to the throne on the death of William's father on 25 October 1760: Cumberland became a very influential advisor to the King and was instrumental in establishing the First Rockingham Ministry.[3] Cabinet meetings were held either at Cumberland Lodge, his home in Windsor, or at Upper Grosvenor Street, his house in London.[3] Cumberland never fully recovered from his wound at Dettingen, and was obese.[3] In August 1760, he suffered a stroke[42] and, on 31 October 1765, he died at Upper Grosvenor Street in London.[3] He was buried beneath the floor of the nave of the Henry VII Lady Chapel in Westminster Abbey.[43] He died unmarried.[3]

27 July 1726 – 31 October 1765:His Royal Highness The Duke of Cumberland[7]

The Duke's full style as proclaimed at his funeral by Garter King-of-Arms was: "the [...] most High, most Mighty, and most Illustrious Prince William Augustus, Duke of Cumberland, and Duke of Brunswick and Lunenburgh, Marquess of Berkhamstead, Earl of Kennington, Viscount Trematon, Baron of the Isle of Alderney, Knight of the most Noble Order of the Garter, and First and Principal Companion of the most Honourable Order of the Bath, third Son of His late most Excellent Majesty King George the Second".[44]

On 20 July 1725, as a grandchild of the sovereign, William was granted use of the arms of the realm, differenced by a label argent of five points, the centre point bearing a cross gules, the first, second, fourth and fifth each bearing a canton gules. On 30 August 1727, as a child of the sovereign, William's difference changed to a label argent of three points, the centre point bearing a cross gules.[47]

There is a memorial Obelisk to the Duke's military services in Windsor Great park. It is inscribed "THIS OBELISK RAISED BY COMMAND OF KING GEORGE THE SECOND COMMEMORATES THE SERVICES OF HIS SON WILLIAM DUKE OF CUMBERLAND THE SUCCESS OF HIS ARMS AND THE GRATITUDE OF HIS FATHER THIS TABLET WAS INSCRIBED BY HIS MAJESTY KING WILLIAM THE FOURTH". According to a local park guide, the Obelisk was originally inscribed "Culloden" but Queen Victoria had "Culloden" removed.[54]

An equestrian statue of the Duke was erected in London's Cavendish Square in 1770, but was removed in 1868 since by that time the 'butcher of Culloden' was generally reviled. The original plinth remained.[55]